Optical filter for wavelength division multipled optical signals

ABSTRACT

Methods for combining or splitting optical signals that include an odd channel set an even channel set. The methods of combining or splitting are performed by subjecting the odd and even channel sets to a first filter that applies phase retardations corresponding with odd integer multiples of half a wavelength for each center wavelength associated with a selected one of the odd and even set of channels and with integer multiples of a full wavelength of each center wavelength associated with a remaining one of the odd set and the even set. The channel sets are then subjected to a second filter that applies phase retardations that are complementary to those experienced by the odd and even set of channels in the first filter. This complementary filtration has the effect of reducing dispersion. These methods may be used in optical devices applicable to a range of telecommunications applications, including optical multiplexers/demultiplexers and optical routers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/944,037, filed Aug. 31, 2001, now U.S. Pat. No. 6,684,002, which is acontinuation-in-part of U.S. patent application Ser. No. 09/879,026,filed Jun. 11, 2001, now U.S. Pat. No. 6,694,066, and which claims thebenefit of U.S. Provisional Patent Application Ser. No. 60/303,705,filed Jul. 5, 2001 and of U.S. Provisional Patent Application Ser. No.60/269,190, filed Feb. 14, 2001. The foregoing patent applications areincorporated herein by reference.

BACKGROUND OF THE INVENTION

-   -   1. The Field of the Invention

The present invention generally relates to optical filters and moreparticularly to optical filers for optical fiber communication networks.

2. Description of the Related Art

The Synchronous Optical Network (SONET) standard defines a hierarchy ofmultiplexing levels and standard protocols which allow efficient use ofthe wide bandwidth of fiber optic cable, while providing a means tomerge lower level DS0 and DS1 signals into a common medium. Currently,optical communication is accomplished by what is known as “wavelengthdivision multiplexing” (WDM), in which separate subscriber/data sessionsmay be handled concurrently on a single optic fiber by means ofmodulation of each of those subscriber data streams on differentportion, a.k.a. channels, of the light spectrum.

The spacing between channels is constantly being reduced as theresolution and signal separation capabilities of multiplexers andde-multiplexers are improved. Current International TelecommunicationsUnion (ITU) specifications call for channel separations of approximately0.4 nm, i.e., 50 GigaHertz (GHz). At this channel separation as many as128 channels may be supported in C-band alone. Each channel is modulatedon a specific center frequency, within the range of 1525-1575 nm, withthe center frequency of each channel provided by a corresponding one of128 semiconductor lasers. The modulated information from each of thesemiconductor lasers is combined (multiplexed) onto a single optic fiberfor transmission. As the length of a fiber increases the signal strengthdecreases. To offset signal attenuation erbium doped fiber amplifiers(EDFAs) are used at selected locations along the communication path toboost signal strength for all the channels. At the receiving end theprocesses is reversed, with all the charnels on a single fiber separated(demultiplexed), and demodulated optically and/or electrically.

Optical filters play important roles in handling these opticalcommunications for the telecommunications industry. They performwavelength multiplexing and demultiplexing of the 128 or more opticalchannels. They may also be used to gain scale EDFAs by Battening theirgain profile.

The requirements for optical filters used for any of these applicationsare very demanding. The close spacing between the channels in a WDM,makes it desirable to design a WDM with flat pass bands in order toincrease the error tolerance. This is primarily because the centerwavelength of a transmitter slips with temperature. Further, thecascading of the WDM stages causes the pass bands to become narrower ateach WDM down the chain. Therefore, the larger the pass bands thegreater the shift tolerance of the channel.

Various devices, such as multi-stage band and comb splitters, have beenproposed to fill these new demanding requirements and none are fullysatisfactory. In a multi-stage band splitter, the first stage makes acoarse split of two wavelength ranges, and subsequent stages make finerand finer splits of sub-bands within each of the wavelength ranges. In amulti-stage comb splitter, the first de-multiplexing stage filters outtwo interlaced periodic sets of relatively narrow band passes and thesubsequent stages employ wider band pass periodic filters until theindividual channels are de-multiplexed. In either case, noise andinter-channel interference are limiting factors in the handling ofincreasingly narrow band pass requirements. Multi-layer thin-filmfilters can be used to construct optical filters in bulk optics, butthey are undesirable because of an increase in the number of layers fornarrow channel spacing, precision of manufacture and expense associatedwith increasingly narrow band pass requirements. Further, dispersionwill become a major issue as channel spacing decreases. Especially at 50GHz channel spacing, dispersion of thin film filter is so big that itcan not be used for OC-192 signal (10 Gbit/sec). Mach-Zehnderinterferometers have been widely employed, but they have a sinusoidalresponse, giving rise to strongly wavelength dependent transmission anda narrow rejection band. Other designs have encountered a variety ofpractical problem.

Accordingly, there is a need for new optical filters for opticalmultiplexing/demultiplexing and other optical applications.

SUMMARY OF THE INVENTION

The present invention provides an optical device that can be used in arange of telecommunications applications including opticalmultiplexers/demultiplexers and optical routers. The optical devicesplits and combines optical signals of frequency division multiplexedoptical communication channels which are evenly spaced apart infrequency from one another. The optical device includes a first filterand a second filter. The first filter splits and combines odd and evenchannels depending on the propagation direction of the optical signal.The first filter exhibits complementary phase retardations correspondingwith odd integer multiples of half a wavelength for each centerwavelength associated with a selected one of the odd and even set ofchannels and with integer multiples of a full wavelength for each centerwavelength associated with a remaining one of the odd set and the evenset. The second filter couples with the first filter to filter the oddand even sets of channels with phase retardations complementary to thoseexperienced by the odd and even set of channels in the first filter.This complementary filtration has the effect of reducing dispersion inthe device.

In an alternate embodiment of the invention the optical deviceinterfaces with a first port communicating odd and even channels andwith second and third ports communicating odd and even channelsrespectively. The optical device includes a linear polarizer, a firstfilter and a second filter. The linear polarizer couples to the firstport for linearly polarizing optical signals. The first filter has afirst free spectral range substantially corresponding with the channelspacing between adjacent odd or even channels. The first filter coupleswith the linear polarizer for splitting and combining odd and evenchannel sets depending on a propagation direction. The first filteroperates as a full waveplate to a selected one of an odd Cannel set andan even channel set and as a half-waveplate to a remaining one of theodd set and the even set. The second filter optically couples with thefirst filter and the second and third ports. The second filter has asecond free spectral range substantially corresponding with the channelspacing between adjacent odd or even channels. The second filter coupleswith the first filter for optical processing of odd and even channelstherewith. The second filter operates as a half-waveplate to theselected one of the odd set and the even set and as a full waveplate tothe remaining one of the odd set and the even set. In an alternateembodiment of the invention a method for splitting and combining opticalsignals is disclosed. The method includes subjecting odd and evenchannel sets to a first set of phase retardations corresponding with oddinteger multiples of half a wavelength for each center wavelengthassociated with a selected one of the odd set of channels and the evenset of channels and corresponding with integer multiples of a fullwavelength for each center wavelength associated with a remaining one ofthe odd set and the even set. The method also includes subjecting oddand even channel sets to a second set of phase retardationscorresponding with integer multiples of a full wavelength retardationfor each center wavelength associated with the selected one of the oddset and the even set and corresponding with odd integer multiples of ahalf wavelength retardation for each center wavelength associated withthe remaining one of the odd set and the even set.

Associated means are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention willbecome ore apparent to those skilled in the art from the followingdetailed description in conjunction with the appended drawings in which:

FIG. 1 is a hardware block diagram of a multiplexer/de-multiplexer(Mux/Demux) fabricated in accordance with the current invention

FIGS. 2A-B are graphs of frequency vs. phase retardation in themux/demux shown in FIG. 1.

FIGS. 3A-B are graphs showing the complementary dispersions profilesabout a representative center frequency of one of the channels.

FIGS. 4A-E are signal diagrams showing filter functions at variouslocations along the optical path of the mux/demux shown in FIG. 1.

FIGS. 5A-B are isometric views showing alternate embodiments of amux/demux shown in FIG. 1.

FIG. 6 is an isometric view of an alternate embodiment of the inventionin which polarization couplers, rather than intensity couplers are usedto split/combine optical signals within the mux/demux.

FIG. 7A is an isometric view showing an alternate embodiment of themux/demux shown in FIG. 6.

FIGS. 7B-C are top/plan and side/elevation views of the mux/demux shownin FIG. 7A

FIG. 7D shows polarization diagrams for light propagating through themux/demux shown in FIGS. 7A-C.

FIG. 8A is an isometric view of an optical filter cell with couplersemploying polarization dependent beam splitting linked by a pair ofdelay paths.

FIG. 8B is an isometric view of an optical filter cell with couplersemploying intensity dependent beam splitting linked by a pair of delaypaths.

FIG. 8C is an isometric view of an optical filter cell shown in FIG. 8Awith one of the optical elements configured for adjustment of the centerwavelength and free spectral range of the filter.

FIG. 8D is an isometric view of an optical filter cell shown in FIG. 8Bwith one of the optical elements configured for adjustment of the centerwavelength and free spectral range of the filter and with an alternatecoupler configuration.

FIG. 8E is an isometric view of an optical filter cell shown in FIG. 8Bwith one of the optical elements configured for adjustment of the centerwavelength and free spectral range of the filter and with an alternatecoupler configuration.

FIGS. 9A-B are isometric and end views respectively of a multi cellimplementation of the optical filter cell shown in FIG. 8A.

FIG. 9C is a side elevation view of the delay paths of the multi-cellimplementation shown in FIGS. 9A-B.

FIG. 9D is a side elevation view of the variable coupling between cellsof the multi cell implementation shown in FIGS. 9A-B.

FIG. 9E shows the individual transforms associated with each of the fourdelay paths through the two cell sequence shown in FIG. 9A

FIG. 10A is an isometric side view of an optical filter constructed froma series of delay paths coupled by intensity dependent beam splitters.FIG. 10B is a side elevation view of the delay paths of the multi-cellimplementation shown in FIG. 10A.

FIG. 10C is a side elevation view of the variable coupling between cellsof the multi-cell implementation shown in FIG. 10A

FIG. 10D shows the individual transforms associated with each of theoptical paths shown in FIG. 10A.

FIG. 11 is a graph showing the pass bards and stop bands associated witha specific filter transform which may be achieved using the opticalfilters of the current invention.

DETAILED DESCRIPTION OF THE EMBODIMENT

FIG. 1 is a hardware block diagram of a multiplexer/de-multiplexer(Mux/Demux) fabricated in accordance with the current invention. Themux/demux is designed to operate on the narrowly spaced frequencydivision multiplexed channels of a telecommunications grid. Thesechannels may be spaced apart in frequency at 50 GHz intervals or less.The mux/demux can depending on the propagation direction of an opticalsignal, split or combine an optical stream with 50 GHz channel spacinginto two separate optical streams with odd and even 100 GHz channelspacing respectively and vice versa. The mux/demux shown in FIG. 1separates/combines odd and even channel sets, with a higher degree ofisolation and a lower dispersion than prior art designs. It may be usedin combination with other stages of similar or different design toseparate out each individual channel of a telecommunications or othercommunication grid.

The mux/demux includes at least two sub-stages 190-192. Sub-stage 190accepts an optical communication signal 102 at an interleave port 104 ofa filter 100 and provides optical outputs in the form of de-interleavedodd and even channel components of the signal at ports 106-108respectively. Sub-stage 192 includes two filters 130, 160 each of asimilar configuration with filter 100. Each filter 130, 160 accepts acorresponding one of the de-interleaved odd and even components fromsub-stage 190 at interleave ports 134, 164 respectively and performsfurther isolation of the odd and even channel components. Filter 130outputs the odd charnel components at port 138 with the remaining port136 serving as a dump port. Filter 160 outputs the even channelcomponents at port 166 with the remaining port 168 serving as a dumpport. The propagation direction of light determines whether the deviceperforms as a multiplexer or de-multiplexer.

Each filter in each sub-stage may include one or more filter cells. Inthe embodiment shown sub-stage 190 includes two filter cells, 110, 120serially coupled to one another between an interleave port 104 andde-interleave ports 106-108 by couplers 112, 118, 122. Each cellincludes a pair of delay paths. Within cell 110 delay paths 114, 116 areshown. One path has a shorter optical pathlength than the other and willbe identified as the fast path, with the other identified as the slowpath. Light traversing the paths will at any given frequency experiencea phase retardation proportionate to the difference in the pathlengthsbetween the slow and fast paths. The couplers 112, 118 define the amountof light which will be split/combined from/to each delay path withincell 110. The couplers 118, 122 define the amount of light which will besplit/combined from/to each delay path within cell 120.

Sub-stage 192 includes filters 130 and 160. Filter 130 includes twofilter cells, 140, 150 serially coupled to one another between aninterleave port 134 and de-interleave ports 136-138 by couplers 142,148, 152. Each cell includes a pair of delay paths. Within cell 140delay paths 144, 146 are shown. The couplers 142, 148 define the amountof light which will be split/combined from/to each delay path withincell 140. The couplers 148, 152 define the amount of light which will besplit/combined from/to each delay path with cell 150. Filter 160includes two filer cells, 170, 180 serially coupled to one anotherbetween an interleave port 164 and de-interleave ports 166-168 bycouplers 172, 178, 182. Each cell includes a pair of delay paths. Withincell 170 delay paths 174, 176 are shown. The couplers 172, 178 definethe amount of light which will be split/combined from/to each delay pathwithin cell 170. The couplers 178, 182 define the amount of light whichwill be split/combined from/to each delay path within cell 180.

Delay paths may be defined by a range of optical elements including:birefringent crystals, semiconductor waveguides and optical fibers forexample. Delay paths may also be formed by discrete optical componentssuch as those shown in the following FIGS. 8-10. Couplers maysplit/combine light on the basis of intensity or polarization forexample.

The combination of first cell and subsequent cells can be looked at asestablishing by the difference between their delay paths the fundamentalsinusoidal harmonic for the sequence and higher order harmonics. One ofthe cells in the sequence, a.k.a the fundamental cell, is designed witha FSR corresponding with the desired fundamental harmonic. Others of thecells, a.k.a the harmonic cell(s) are designed with FSRs which areinteger factions of the base FSR. The coefficients or amplitude of eachharmonic are provided by varying the couplingratio/percentage/coefficients between the cells. Whether a sub-stageincludes within a filter a single filter cell or multiple seriallycoupled filter cells, at least one of the cells i.e. the fundamentalcell, in each filter exhibits a free spectral range (FSR) whichcorresponds with the periodic frequency spacing between the odd or evenchannels, e.g. 100 GHzThe optical path length difference between the twodelay paths in a filter, corresponds inversely with the free spectralrange (FSR) exhibited by the filter. This relationship is set forth inthe following Equation 1. $\begin{matrix}{{FSR} = \left( \frac{c}{{L_{I} - L_{J}}} \right)} & \text{Equation~~~1}\end{matrix}$where L_(I) and L_(I) are the total optical path length of each of thedelay paths. In each filter of a sub-stage additional filter cells,formed from delay path pairs may be serially coupled to one another.Where a filter includes more than one filter cell the delay paths formedthereby are serially coupled to provide a plurality of distinct delaysalong a plurality of combinations of optical paths from the input to theoutput of the stage. The spectral characteristics of the output beam(s)formed thereby correspond to the sum of a Fourier series in which eachterm corresponds in amplitude and phase with each of the plurality ofcombinations of optical paths between the input and the output(s). Thefundamental cell provides the fundamental frequency componentcorresponding with the spacing between adjacent odd and adjacent evenchannels. Additional cells may provide the harmonics, required forestablishing a flat top composite comb filter function for the mux/demuxsuch as that shown in FIG. 4E.

The mux/demux shows improvements in dispersion over prior art designs asa result of an optical pathlength shift between the fundamental cell,e.g. cell 110, in the first sub-stage 190 and the fundamental cell(s) inthe second sub-stage 192, e.g. cells 140, 170. The fundamental cell(s)in the second sub-stage have an optical pathlength difference shiftedfrom the optical pathlength difference of the fundamental cell in thefirst stage by odd integer multiples of one-half of the wavelength ofinterest as shown in the following Equation 2: $\begin{matrix}{{{OPD}_{F1} + {\left( {{2\quad N} + 1} \right)\quad\frac{\lambda}{2}}} = {OPD}_{F2}} & \text{Equation~~~2}\end{matrix}$

where OPD_(F1) is the optical pathlength difference of the fundamentalcell in the first sub-stage and OPD_(F2) is the optical pathlengthdifference of the fundamental cell(s) in the second sub-stage. Thisshift results in substantially complementary dispersion profiles fortheft and second sub-stages 190-192, the net effect of which is anormalization of dispersion in each communication channel and aconcomitant improvement in signal integrity each telecommunicationschannel as shown in FIGS. 3A-B. This shift is achieved with a negligibledeviation, e.g. less than 0.3%, between the FSR of the fundamental cell110 of sub-stage 190 and the FSRs of the fundamental cells 140, 170 ofsub-stage 192.

FIGS. 2A-8 are graphs of frequency, expressed in terms of increasingchannel number (y axis) vs. phase retardation (x axis) between the twodelay paths for the fundamental cell(s) 110, and 130, 160 in thesub-stages 190-192 respectively. Frequency is represented by channelnumber with channels 9-13 shown and with each channel having a fixedcenter frequency/wavelength. Channels 9-10 might for example be spacedapart by a 50 GHz interval and be centered on 1549.315 nm and 1550.116nm wavelengths. The relationship between phase shift and frequency isshown in the following Equation 3: $\begin{matrix}{{PhaseShift} \equiv {2\quad\pi\quad\frac{\Delta\quad L}{\lambda_{0}}} \equiv {2\quad\pi\quad f\quad\frac{\Delta\quad L}{c}}} & \text{Equation~~~3}\end{matrix}$

where ΔL is the optical pathlength difference, f is the frequency and cis the speed of light in free space. The phase shift increases linearlywith optical pathlength difference and with frequency. In the exampleshown channel numbers, e.g., Cb 9-Ch 13, are evenly spaced apart infrequency, thus in this example the vertical was corresponds withfrequency, expressed in terms of channel number. The linear relationshipbetween phase retardation and frequency is shown in line 200 whichbisects the x and y axis. The mux/demux fundamental cells in the fistand second stages are designed to subject the odd channel centerfrequencies and the even channel center wavelengths to asymmetricalphase shifts both within the cell as well as between stages. The graphsshown in FIGS. 2A-B show one of two possible phase shift relationshipsfor odd and even wavelengths.

FIG. 2A shows phase retardation in the first sub-stage for fundamentalcell 110. The intersects of Channels 9-12 with line 200 are shownprojected onto the x axis. The intersect 202 for Channel 10 and 204 forChannel 11 are explicitly referenced. Odd channels in this example,experience phase retardations of half that of the incident wavelength orodd integer multiples thereof. Thus Channels 9 and 11 are shownexperiencing absolute phase retardations across the slow path ascompared to the fast path of 180° and 540° respectively. Even channelsin this example, experience phase retardations equal to the incidentwavelength or integer multiples thereof. Thus Channels 10 and 12 areshown experiencing absolute phase retardations across the slow path ascompared to the fast path of 360° and 720° respectively

FIG. 2B shows phase retardation in the second sub-stage for either ofthe fundamental cells 140 and 170. The retardations for the odd channelsand even channels are now shifted so as to experience a retardationcomplementary to that experienced in the first stage. The intersect 206for Channel 10 and 208 for Channel 11 are shown. Odd channels in thesecond sub-stage fundamental cell experience phase retardations of theincident wavelength or integer multiples thereof. Thus Channels 11 and13 are shown experiencing absolute phase retardations across the slowpath as compared to the fast path of 380° and 720° respectively. Evenchannels in this example, experience phase retardations equal to halfthe incident wavelength or odd integer multiples thereof. Thus Channels10 and 12 are shown experiencing absolute phase retardations across theslow path as compared to the fast path of 180° and 540° respectively.

In an alternate embodiment of the invention the asymmetry may bereversed with the even channels experience phase retardations of halfthat of the incident wavelength or odd integer multiples thereof in thefirst sub-stage fundamental cell and retardations of the incidentwavelength or integer multiples thereof in the fundamental cell of thesecond sub-stage. Conversely, in this alternate embodiment the oddchannels experience phase retardations of the incident wavelength orinteger multiples thereof in the first sub-stage fundamental cell andretardations of the half the incident wavelength or odd integermultiples thereof in the fundamental cell of the second sub-stage.

FIGS. 3A-B are graphs showing the complementary dispersion profilesabout a representative center frequency of one of the channels. FIG. 3Ashows a representative dispersion profile where coupling of light ontofast and slow paths is in equal proportions. The dispersion profiles 300and 302 for the individual sub-stages are shown along with thesubstantially flat line composite dispersion 304. The flat linedispersion profile which results from the asymmetrical phase retardationin the fundamental cells of the first and second sub-stages isadvantageous because it improves the signal integrity associated withmultiplexing and de-multiplexing telecom communications.

FIG. 3B shows a representative dispersion profile where coupling oflight onto fast and slow paths is in unequal proportions. The dispersionprofiles 310 and 314 for the individual sub-stages are shown along withthe composite dispersion 316. The composite dispersion exhibits somedeviation from the desired flat line response but the tradeoff in termsof enhanced stop bends in the filter transform is appropriate for someapplications as will be shown in the following FIGS. 4A-E.

FIGS. 4A-E are sign diagrams showing filter functions at variouslocations along the optical path of the mux/demux shown in FIG. 1. Inthis embodiment of the invention couplers 112, 118, 122 and 142, 148,152 and 172, 178, 182 couple light asymmetrically onto the fist and slowpats of each cell.

The signal diagrams shown in FIGS. 4A-B show the different comb filterfunctions to which the even channels are exposed in the first sub-stageand the second sub-stage respectively. The first comb filter function towhich the even channels are exposed in the first sub-stage includespassbands for the even channels interlaced with residual components, orbleed through, of the odd channels and is shown in FIG. 4A. In the firstsub-stage, in this example the even channels are subject to a phaseretardation substantially equal to the incident wavelength or integermultiples thereof. Thus there is a passband 160 for channel 10 and onefor channel 12. The center frequency 164 for the passband for channel 12coincides with a selected order of the incident wavelength, e.g. order3875. Between the passbands for the even channels there is a bleedthrough of the odd passbands below the −10 dB level. The bleed through162 for channel 11, as well as channels 9 and 13 are shown. This bleedthrough results from asymmetric coupling of light onto the fast and slowpaths in amounts other than 50%/50%.

The coupling asymmetries in the first filter between the fast and slowpaths of each filter cell are continued within each filter and thefilter cells thereof in the second sub-stage as shown for the evenchannels in FIG. 4B. Because of the wavelength shift of λ/2 or oddinteger multiples thereof, in the optical pathlength difference betweenthe fundamental cells of the first sub-stage and the second sub-stage,the even channels are subject to a second comb filter function differentthan that to which they were exposed in the first sub-stage. This secondcomb filter function shown in FIG. 4B includes narrow stop bands, andsubstantially attenuated bleed-through of the odd channels. There is apassband 166 for channel 10 and one for channel 12 with a slight dip inthe flat top. The center frequency 164 for channel 12 coincides with adifferent selected order of the incident wavelength, e.g. order 3876than was the case in the filter of the fist sub-stage as shown in FIG.4A

The signal diagrams shown in FIGS. 4C-D show the comb filter functionsto which the odd channels are exposed in the first sub-stage and thesecond sub-stage respectively. In the fist sub-stage, in this examplethe odd channels are subject to the second comb filter function with awavelength shift of λ/2 or odd integer multiples thereof. Thus there isa passband 170 for channel 11 and one for channels 9 and 13. The centerfrequency 164 for the passband for channel 12 coincides with a selectedorder of the incident wavelength, e.g. order 3875. The filter functionfor the odd channels in the first sub-stage exhibits narrow stop bands,and substantially attenuated bleed-through. The coupling asymmetries inthe first filter between the fast and slow paths of each filter cell arecontinued within the filter(s) and the filter cells thereof in thesecond sub-stage.

As show in FIG. 4D the wavelength shift of λ/2 in the optical pathlengthdifference between the fundamental cells of the first sub-stage and thesecond sub-stage results in the odd channels also being subject to adifferent, i.e. complementary filter function to that experienced in thefirst sub-stage. The odd channels are exposed to the first comb filterfunction with a wavelength shift of λ/2 or odd integer multiplesthereof. There is a passband 174 for channel 11 and one for channels 9,13. Between the passbands for the odd channels there is a bleed throughof the even passbands below the −10 dB level. The bleed through 172 forchannel 10, as well as channel 12 is shown. This bleed through resultsfrom asymmetric coupling of light onto the fast and slow paths inamounts other than 50%/50%. The sane coupling ratios used in the firstsub-stage are applied in the second stage. The center frequency 164 forthe passband for channel 12 coincides with a different selected order ofthe incident wavelength, e.g. order 3876 than was the case in the filterof the first sub-stage as shown in FIG. 4C.

FIG. 4E shows the composite performance for the mux/demux for both theodd and even channels. The passband 210 for even channel 10 as well asfor channel 12 is shown. The passband 212 for odd channel 11 as well asfor channels 9, 13 are shown. Each passband exhibits steep side profilesand broad stopbands when compared with prior art designs. The passband212 for channel 11 is shown with a broad fiat top 204 and with broadpassbands 216-218. Supposed on the passband 212 is a skirt 220representative of traditional passband profiles. By comparison thecurrent mux/demux exhibits a significant improvement in the passbandprofiles it generates with relatively steeper sides and broaderstopbands. These improvements translate into increases in the signalintegrity of the telecommunications data handled by the mux/demux

FIGS. 5A-B are isometric views showing alternate embodiments of amux/demux shown in FIG. 1. In this embodiment of the invention couplerssplit light on the basis of intensity between fast and slow delay pathswithin each filter cell. The delay path pairs in these embodiments maybe fabricated from optical fibers, semiconductor waveguides, or discretemicro-optic elements for example. Examples of the latter are describedand discussed in connection with the following FIGS. 8, 10. Theintensity couplers may be fabricated from fused optical fibers, withinwaveguides, or as dielectric coatings on optical elements for example.Examples of the latter are described and discussed in connection withthe following FIGS. 8, 10.

In FIG. 5A the first sub-stage includes filter 500 and the secondsub-stage includes filters 530 and 560. Filter 500 accepts an opticalcommunication signal 102 at an interleave port 504 and provides opticaloutputs in the form of de-interleaved odd and even channel components ofthe signal at ports 106-108 respectively. The second sub-stage includestwo filters 530, 560. Each filter 530, 560 accepts a corresponding oneof the de-interleaved odd and even components from the filter 500 of thefirst sub stage at interleave ports 534, 564 respectively and performsfor further isolation of the odd and even channel components. Filter 530outputs the odd channel components at port 138 with the renaming port136 serving as a dump port. Filter 560 outputs the even channelcomponents at port 166 with the remaining port 168 serving as a dumpport. The propagation direction of light determines whether the deviceperform as a multiplexer or de-multiplexer.

Each filter in each sub-stage may include one or more filter cells. Inthe embodiment shown the filter 500 of the first sub-stage includes twofilter cells, 510, 520 serially coupled to one another between theinterleave port 504 and de-interleave ports 506-508 by couplers 512,518, 522. Each filter cell includes a pair of delay paths. Within filtercell 510 delay paths 514, 516 are shown. One path has a shorter opticalpathlength than the other and will be identified as the fast path, withthe other identified as the slow path. Optical pathlength is a productof the physical distance “d” of an optical path and the index ofrefraction “n” along the path. If path 514 and 516 are made from thesame optical material then path 514 will have the longer opticalpathlength and will thus be identified as the slow path. Path 516 wouldin therefore be identified as the fast path. Light traversing the pathswill at any given frequency experience a phase retardation proportionateto the difference in the pathlengths between the slow and fast paths.The couplers 512, 518 define the amount of light which will besplit/combined from/to each delay path within cell 510. The couplers518, 522 define the amount of light which will be split/combined from/toeach delay path 524, 526 within cell 520.

The second sub-stage includes filters 530 and 560. Filter 530 includestwo filter cells, 540, 550 serially coupled to one another between aninterleave port 534 and de-interleave ports 136-138 by couplers 542,548, 552. Each cell includes a pair of delay paths. The couplers 542,548 define the amount of light which will be split/combined from/to eachdelay path within cell 540. In an embodiment of the invention thecoupling ratios are the same as the coupling ratios for the filter 500of the first stage. The couplers 548, 552 define the amount of lightwhich will be split/combined from/to each delay path within cell 550.Filter 560 includes two filter cells, 570, 580 serially coupled to oneanother between an interleave port 564 and de-interleave ports 166-168by couplers 572, 578, 582. Each cell includes a pair of delay paths. Thecouplers 572, 578 define the amount of light which will besplit/combined from/to each delay path with cell 570. The couplers 578,582 define the amount of light which will be split/combined from/to eachdelay path within cell 580.

Delay paths may be defined by a range of optical elements includingsemiconductor waveguides and optical fibers for example. Delay paths mayalso be formed by discrete optical components such as those shown in thefollowing FIGS. 8-10. The couplers may which split/combine light on thebasis of intensity may be fabricated from transmissive/reflectivedielectric coatings. Whether a sub-stage includes within a filter asingle filter cell or multiple serially coupled filter cells, at leastone of the cells, a.k.a. a fundamental cell, in each filter exhibits afree spectral range (FSR) which corresponds with the periodic frequencyspacing between the odd or even channels, e.g. 100 GHz. In theembodiment shown in FIG. 5A cell 510 in the first sub-stage and cells540, 570 in the second stage might for example serve as the fundamentalcells. The FSR of the second sub-stages fundamental cell(s) is adjustedby a fractional percent, or more particularly by an amount which resultsin the optical pathlength difference in the fundamental cell(s) of thesecond sub-stage which diverges from that of the fundamental cell of thefirst sub-stage by half the incident wavelength. This divergence resultsin the shift discussed above in connection with FIGS. 2-4, which widensthe stopbands of exhibited by the overall device.

FIG. 5B is an isometric view showing an alternate embodiment of themux/demux shown in FIG. 5A in which the second sub-stage comprises asingle filter 584 coupled to both the de-interleaved ports 106, 108 ofthe first sub-stage filter 500. Filter 584 includes two interleavedinput ports 586, 588 and four do-interleaved output ports 166, 138 and136, 168. The latter two ports, 136, 168 are dump ports. The opticalsignals with both odd and even channel components are processed alongparallel optical paths within the filter 584. The filter includes twofilter cells 590, 592. Light is split/combined along the fast and slowpath of the first filter cell 590 via couplers 594, 596. Light issplit/combined along the fast and slow paths of the second filter cell592 by the couplers 596, 598.

In an embodiment of the invention suitable coupling percentages onto thefast and slow paths are determined by the transmission/reflection ratiosfor each of couplers 512, 518, 522 in the first filter. Symmetriccoupling would for be satisfied by couplers with 50%/50%transmission/reflection. For asymmetrical coupling in accordance withthe current invention the following Table 1 sets forth two among manyacceptable asymmetric coupling possibilities for embodiments of theinvention in which each sub-stage includes within a correspondingfilter(s) two filter cells, interlaced with three couplers such as thoseshown in either FIGS. 5A-B.

TABLE 1 Input Inten- 1^(st) Cplr Slow % 2^(nd) Cplr Slow % 3^(rd) CplrSlow % sity % Tx Fast % % Tx Fast % % Tx Fast % Case 1 1 67% 67% 28% 28%85% 85% 33% 72% 15% Case 2 1 59% 59% 30% 30% 91% 91% 41% 70%  9%

FIG. 6 is an isometric view of an alternate embodiment of the inventionin which polarization couplers, rather than intensity couplers are usedto split/combine optical signals within the mux/demux. The delay pathpairs in this embodiment may be fabricated from optical fibers,semiconductor waveguides, or discrete micro-optic elements for example.Examples of the latter are described and discussed in connection withthe following FIGS. 8, 9. The polarization couplers ma be fabricatedfrom birefringent crystals or as dielectric coat on optical elements forexample. Examples of the latter are described and discussed inconnection with the following FIGS. 8, 9.

In FIG. 6 the first sub-stage includes filter 600 and the secondsub-stage includes filters 630 and 66. Filter 600 accepts an opticalcommunication in the form of a linearly polarized input signal 102 at anangle φ₁ with respect to the optical axis at coupler 612. The opticalaxis may coincide with one or the other of the fast and slow axis. Thefilter 600 provides optical outputs in the form of de-interleaved odd106 and even 108 channel components of the input signal. Each filter630, 660 of the second sub-stage accepts a corresponding one of thede-interleaved odd and even components from the filter 600 of the firstsub-stage at polarization couplers 642, 672 respectively and performsfurther isolation of the odd and even channel components. The odd andeven channel components are also communicated with the correspondingfilter of the second stage in the form of linearly polarized light withinput vectors of φ and 0.5π+φ with respect to the couplers 642, 672respectively. Filter 630 outputs the odd channel components at port 138with the remaining port 136 serving as a dump port. Filter 660 outputsthe even channel components at port 166 with the remaining port 168serving as a dump port. The propagation direction of light determineswhether the device performs as a multiplexer or demultiplexer.

Each filter in each sub-stage may include one or more filter cells. Inthe embodiment shown the filter 600 of the first sub-stage includes twofilter cells, 610, 620 serially coupled to one another by couplers 612and 618. Each filter cell includes a pair of delay paths. Within filtercell 610 delay paths 614, 616 are shown. One path has a shorter opticalpathlength than the other and will be identified as the fist path, withthe other identified as the slow path. Within cell 620 fast 624 and slow626 delay paths are also shown. Optical pathlength is a product of thephysical distance “d” of an optical path and the index of refraction “n”along the path. If path 614 and 616 are made from the same opticalmaterial as in the case where a birefringent crystal forms the filtercell 610, then whichever of the “e” or “o” ray experiences a higherindex of refraction along the optical path will be characterized as theslow path. Light traversing the paths will at any given frequencyexperience a phase retardation proportionate to the difference in thepathlengths between the slow and fast paths. The amount of light coupledonto the slow and fast paths 614, 616 in cell 610 is determined by theangle φ of the input vector with respect to the optical axis. Where thecell is fabricated from a birefringent crystal the coupler, slow andfast axis are integral with the optical axis defined by the crystallinestructure of the birefringent crystal. Where the cell if formed from apolarization coupler and fast and slow paths discrete from thepolarization coupler then the optical axis is the optical axis of thecoupler. In the embodiment shown light propagating in the forwarddirection from input 102 couples with the first cell at an angle φ₁ withrespect to the optical axis of the coupler 612 and/or cell 610 and at anangle φ₂ with respect to the optical axis of the coupler 618 and/or cell620.

The second sub-stage includes filters 630 and 660. Filter 630 includestwo filter calls, 640, 650 serial coupled to one another by couplers 642and 648. Each serially coupled to one another by couplers 672 and 678.Each cell includes a pair of delay paths. The couplers of each filter inthe second stage generally exhibit the same coupling ratios as thecouplers in the filter of the first sub-stage. Del paths may be definedby a range of optical elements including: birefringent crystals,semiconductor waveguides and optical fibers for example. Delay paths mayalso be formed by discrete optical components such as those shown in thefollowing FIGS. 8-9. The couplers may be fabricated from birefringentcrystals, or polarization sensitive dielectric coatings, such as thosediscussed in connection with FIGS. 8-9.

Whether a sub-stage includes within a filter a single filter cell ormultiple serialy coupled filter cells, at least one of the cells, a.k.aa fundamental cell, in each filter exhibits a free spectral range (FSR)which corresponds with the periodic frequency spacing between the odd oreven channels, e.g. 100 GHz. The fundamental cell may in a multi-cellembodiment be placed in any sequence with respect to the other cells ofthe filter. In the embodiment shown in FIG. 6, cell 610 in the firstsub-stage and cells 640, 670 in the second stage might for example serveas the fundamental cells. The FSR of the second sub-stages fundamentalcell(s) is adjusted by a fractional percent, or more particularly by anamount which results in an optical pathlength difference in thefundamental cell(s) of the second sub-stage which diverges from that ofthe fundamental cell of the first sub-stage by half the incidentwavelength. This divergence results in the shift discussed above inconnection with FIGS. 2-4, which widens the stopbands of exhibited bythe overall device.

The odd and even channel sets within an optical signal experience thefiltering of the fundamental cell 610 of the first sub-stagedifferently. The fundamental cell of the first sub-stage operates as afull waveplate to a selected one of an odd channel set and an evenchannel set and as a half-waveplate to a remaining one of the odd setand the even set. Within the fundamental cell(s) 640, 670 of the secondsub-stage the opposite filtration effect is experienced by the odd andeven channel sets. The fundamental cell(s) of the second sub-stageoperate as a half-waveplate to the selected one of the odd set and theeven set and as a full waveplate to the remaining one of the odd set andthe even set. Where a selected channel set odd/even experiences thefundamental cell as a half waveplate linearly polarized light withfrequency components associated with the selected channel set whichenters the cell with one polarization vector, emerges from the cell witha relative phase shift of ½ π and a rotation in the associatedpolarization vector in the direction of the optical axis of the couplerin an amount 2φ where φ is the angle between the input polarizationvector and the optic axis of the cell. Where a selected channel setodd/even experiences the fundamental cell as a full waveplate linearlypolarized fight with frequency components associated with the selectedchannel set which enters the cell with one polarization vector, emergesfrom the cell with the same polarization vector.

In the embodiments of the invention shown in FIG. 6 as well as in FIGS.7A-D suitable coupling percentages onto the fast and slow paths aredetermined by the angle φ between the input polarization vector and theoptic axis of the cell/coupler. The following Table 2 shows suitableangles for both symmetric (Case 1) and asymmetric coupling (Cases 2-3)for embodiments of the invention in which each sub-stage includes withina corresponding filter(s) two filter cells and corresponding couplers asshown in FIG. 6.

TABLE 2 Input Inten- 1^(st) Cplr Slow % 2^(nd) Cplr Slow % 3^(rd) CplrSlow % sity φ Fast % φ Fast % φ Fast % Case 1 1 45° 50% −15° 25% 0° 93%Sym- 50% 75%  7% metric Case 2 1 40° 59% −17° 30% 0° 91% Asym- 41% 70% 9% metric Case 3 1 35° 67% −22.5°   29% 0° 85% Asym- 33% 71% 15% metric

FIG. 7A is an isometric view showing an alternate embodiment of themux/demux shown in FIG. 6. FIGS. 7B-C are top/plan and side/elevationviews of the mux/demux. FIG. 7D shows polarization diagrams 776-798 forlight propagating through the mux/demux. In this embodiment of theinvention the second sub-stage comprises a single filter 730 opticallycoupled with the polarized outputs of the first sub-stage filter 700.

On the forward/demux path an optical signal 102 may be introduced via anoptical fiber into lens 702. Lens 702 may include a Graded Index ofRefraction (GRIN) or other lens system. The lens collimates/focuseslight depending on the propagation direction from/to the beam splitter704. The beam splitter may be fabricated from a birefringent crystalwith an optic axis oriented to effect a walk-off of the forward beamonto waveplates 706-708. The waveplates are broadband and haveoptical-axis oriented to effect a linearization of the polarizationvectors of the two rays formed by the beam splitter. The linearly polarrays are then introduced into the filter 730 of the first sub-stage 700and specifically into cell 710 thereof. Cell 710 is, in this embodimentof the invention, fabricated from dielectric polarization dependentcouplers on opposite ends of a pair of fast and slow delay paths. Thegeneral cell structure for this and subsequent filter cells, 720, 740,750 is set forth in detail in the following FIGS. 8-9. Light is coupledbetween filter cell 710 and filter cell 720. The rotation angle φ of thefilter cells 710, 720 with respect to the input polarization vector ofeach of the two rays into which the optical signal has been splitdetermines the coupling of light onto the fast and slow paths of eachcell. Odd and even signal components are output by filter cell 720. Thepassbands for the odd and even channel components at this pointcorrespond with the passbands discussed above in connection with FIGS.2A, 4A and 4E.

Between the first sub-stage and the second sub-stage a beam splitter 722and waveplates 724-726 are positioned. The beam splitter splits the oddand even signal outputs from the first sub-stage into component vectorswhich are then polarized by waveplates 724-726. The component vectorsfor the odd and even channels are then introduced into the secondsub-stage 730 where they will be further isolated. The second sub-stageincludes two filter cells 740, 750 serially coupled with one another.One of these cells a.k.a the fundamental cell, as also one of the cellsof the first stage, exhibits a free spectral range corresponding withthe channel spacing between adjacent odd or even channels. Thefundamental cell of the first sub-stage and the second sub-stage divergefrom one another in optical pathlength difference by the above discussedamount of ½ A the incident wavelength. This produces the shift whichresults in dispersion reduction/compensation. Light propagating from thefilter cell 750 of the second sub-stage propagates through an opticalbean bender 762 formed from an inclined surface. This has the effect oflinearizing the rays as they are presented to the combined waveplates764 and splitter/combiner 766. The waveplates orthogonalize the odd andeven channel components and pass them to the splitter/combiner. On theforward/demux path this results in two arbitrarily polarized beamscorresponding with the de-interleaved odd and even channel signalcomponents. These are focused via lens 768 onto optical fiber or othersuitable connectors which accept the odd and even channel components138, 166 respectively.

In the embodiment of the invention shown in FIGS. 7A-D the odd and evenchannel sets experience the first sub-stage and second sub-stagedifferently. The fundamental cell, e.g. cell 720, of the first sub-stage700 operates as a full waveplate to a selected one of an odd channel setand an even channel set and as a half-waveplate to a selected one of theodd set and the even set. Within the fundamental cell, e.g. cell 750 ofthe second sub-stage 730 the opposite filtration effect is experiencedby the odd and even channel sets. The fundamental cell of the secondsub-stage operates as a half-waveplate to the selected one of the oddset and the even set and as a full waveplate to the remaining one of theodd set and the even set.

FIG. 8A is an isometric view of an optical filter cell 1100 withcouplers employing polarization dependent beam splitting linked by apair of delay paths 1150 and 1146, 1148, 1152. Each coupler transmitsand reflects light depending on the input properties of the light. Inthe embodiment of the invention shown in FIG. 8A, each coupler ispolarization sensitive and includes “P” and “S” polarization axisorthogonal to one another. A first coupler is positioned in thepropagation path of incoming polarized light and transmits and reflectscomponents of incoming polarized light aligned with the “P” and “S”polarization axis of the coupler respectively. Light transit andreflected by the coupler follows two distinct delay paths, one fortransmitted light and the other for reflected light. Where incominglight is orthogonally polarized, the first coupler provides configurableamounts of coupling and cross-coupling of each of the orthogonalpolarization vectors of the incoming light with either of the pair ofdelay paths. A second coupler positioned at a location where the twodistinct delay paths intersect reverses the process and realigns lightwith orthogonal polarization vectors onto a common propagation axis.

The cell is shown within an “x,y,z” Cartesian coordinate system. Thecell includes opposing optical glass blocks 1110, 1130 displaced fromone another along the z axis with the optical elements 1120A-B coveringthe span between the blocks. Optical glass block 1110 is shown with areflector 1112 and a polarization dependent beam splitter 1114 displacedfrom each other in a direction defined by the y axis. Optical glassblock 1130 is shown with a reflector 1132 and a polarization dependentbeam splitter 134 displaced from each other in a direction defined bythe y axis. The polarization dependent beam splitters have “S”polarization axes 1116 and 1136 respectively which are aligned with oneanother and in the orientation of the cell that is shown, also alignedwith the x axis. The “P” polarization axis of each polarizationdependent beam splitter are orthogonal to the “S” axis. Polarized lightinput at the first port 1102 will couple with both the P and S axis ofthe fist coupler 1114, a polarization beam splitter in amounts whichcorresponded with the relative angular rotation between the polarizationvector(s) of the polarized input and the orthogonal P and S polarizationaxis of the beam splitter. The component of a polarized input whichprojects onto the S polarization axis of the beam splitter will bereflected by the beam splitter. The component of a polarized input whichprojects onto the P polarization axis of the beam splitter will betransmitted by the beam splitter. Between the couplers an opticalelement 1120 is shown.

Each optical glass block 1110, 1130 may in fact be fabricated from twopairs of prism. In the case of block 110 the polarization dependent beamsplitter 114 may be formed from a first pair of prisms at right or othercomplementary angles to one another. These may be affixed to oneanother, e.g. cemented, to minimize wave front distortion. Thehypotenuse of one of the prisms is coated with a multi layer dielectricpolarizing beam splitter coating. The prisms are then affixed to oneanother, to form a first rectangle, the exterior surfaces of which maybe antireflection coated (AR) to minimize surface reflection losses. Asecond pair of prisms may be used to form the reflector 1112. Thehypotenuse of one of this second pair of prisms is coated with areflective dielectric coating, and cemented to the hypotenuse of theother of the second pair of prisms. The hypotenuses of this second pairof prisms are then affixed to one another as well to form a secondrectangle, the exterior surfs of which may also be AR coated. The tworectangles formed by the two pairs of prisms may then be affixed to oneanother to form the glass block 1110. A similar technique may be used tofabricate the second block 130.

The cell couples light bi-directionally between a first port 1102 and asecond port 1104 displaced from one another in a direction defined bythe z axis. For purposes of illustration, polarized light 1140 is shownentering the cell at the first port and exiting as a polarized outputbeam 1154 at the second port though the propagation in the oppositedirection is also supported. The cell is also highly directional so thatlight propagating in one direction is independent of the lightpropagating in the reverse direction. The polarized light beam 1140 maybe arbitrarily, circularly or linearly polarized. In the example shown,beam 1140 is linearly polarized with a polarization vector 1144 at anangle φ₁ with respect to the “S” polarization axis 1116 of the cell. Asthe beam 1140 enters the cell it is accepted onto either of two distinctP and S delay paths 1150 and 1146, 1148, 1152 respectively. These delaypaths link the polarization dependent beam splitters 1114, 1134. Theamount of light that is coupled onto either delay path is determined bythe angle φ₁ of the incoming beam vector with respect to the Spolarization axis of the cell. In the example shown, light frompolarization vector 1144 in amounts proportionate to sin(φ₁) and cos(φ₁)will couple to the P and S delay paths respectively. Rotation of thecell about the propagation path, e.g. the z axis, of the beam 1140 canbe used to vary the coupling percentages or ratios between the incominglight and the P and S delay paths. Where incoming light includesorthogonal polarization vectors the coupling of either vector will bedetermined by projecting that vector onto the P and S polarization axisof the polarization beam splitter as well. The polarization beamsplitters 1114, 1134 thus serve as couplers which provide configurableamounts of coupling and cross-coupling of incoming beams with either ofthe pair of delay paths.

The amount of delay on the P and S delay paths are θ_(P1) and θ_(S1)respectively. The delay of each path is determined by its correspondingoptical path length. The optical path length of each path is the sum ofthe product of physical dimension and the index of refraction of all theoptical elements on each of the two distinct P and S delay paths 1150and 1146, 1148, 1152 respectively. The delay difference for the cell isproportional to the difference in the optical path lengths between the Pand S delay paths. The delay difference exhibits itself in the opticalproperties of the output beam 1154. That output beam exhibits theinterference pattern produced by the re-coupling of the P and S delaypaths by the second of the polarization beam splitters 1134 into asingle output beam. The output beam includes orthogonal polarizationvectors 156-1158. Each contains complementary periodic stop bands andpass bands of the other with center wavelengths the spacing betweenwhich is inversely related to the delay difference between the P and Sdelay paths. In other words the larger the delay difference the smallerthe wavelength spacing which the optical filter cell can resolve. Thefirst vector 1156 contains pass bands with center wavelengths at eveninteger multiples of the periodic interval-established by the delaydifference. The second vector 1158 contains pass bands with centerwavelengths at odd integer multiples of the periodic intervalestablished by the delay difference.

The cell nay be provided with an appropriate lens, e.g. a GRIN lens orother suitable lens, and a linear polarizer coupled to the first port tolinearly polarize arbitrarily polarized incoming light and to direct itto the first port. At the opposite end, a beam displacer/combiner may becoupled with the second port to displace and combine orthogonallypolarized odd and even channel components 1156-1158 of an optical beamdepending on the propagation direction of the light beam. An appropriatelens(es), e.g. GRIN, may also be added at this end to focus andcollimate the light from the beam displacer/combiner depending on thepropagation direction. The resultant system may serve as either or botha multiplexer or a demultiplexer depending on a propagation direction ofthe light.

In an alternate embodiment of the invention there may be a singlereflector replacing reflectors 1112 and 1130 to bend the S delay pathbetween the two polarization beam splitters 1114 and 1134. In stillanother embodiment additional reflectors may be added.

In an alternate embodiment of the invention an opposing pair ofback-to-back birefringent crystals may be used instead of thepolarization beam splitters to split incoming light into an “e” and an“o” ray delay path corresponding with “S” and “P” delay pathsrespectively. The principal planes of the pair of crystals would bealigned in a common plane with the optical axis of each birefringentcrystal at substantially complementary angles to one another so as tocause a splitting and recombining of the e and o ray delay paths.

FIG. 8B is an isometric view of an optical filter cell 1106 withcouplers employing intensity dependent beam splitting linked by a pairof delay paths 1170 and 1166, 1168, 1172. This cell is also shown withinan. “x,y,z” Cartesian coordinate system. The cell includes many of thefeatures of the cell shown in FIG. 8A with the exception that thecoupling function is here performed by partial reflectors 1164, 1174which form intensity beam splitters. The cell includes opposing opticalglass blocks 1110, 1130 displaced from one another along the z axis withthe optical element 1120 covering the span between the blocks. Opticalglass block 1110 is shown with a reflector 1112 and the intensity beamsplitter 1164 displaced from each other in a direction defined by the yaxis. Optical glass block 1130 is shown with a reflector 1132 and anintensity beam splitter 1174 displaced from each other in a directiondefined by the y axis. Between the couplers an optical element 1120 isshown.

Each optical glass block may in fact be fabricated from two pairs ofprisms. In the case of block 1110 the intensity beam splitter 1164 maybe formed from a first pair of prisms at right or other complementaryangles to one another, which are cemented together to minimize wavefront distortion. The hypotenuse of one of the prisms is coated with amulti layer dielectric beam splitter coating which exhibits configurableamounts of transmission and reflection of an incident beam. The prismsare then affixed to one another, e.g. cemented together, to form a firstrectangle, the exterior surfaces of which may be antireflection coated(AR) to minimize surface reflection losses. A second pair of prisms maybe used to form the reflector 1112. The hypotenuse of one of this secondpair of prisms is coated with a reflective dielectric coating, andaffixed to the hypotenuse of the other of the second pair of prisms. Thehypotenuses of this second pair of prisms are then affixed to oneanother as well to form a second rectangle, the exterior surfaces ofwhich may also be AR coated. The two rectangles formed by the two pairsof prisms may then be affixed to one another to form the glass block1110. A similar technique may be used to fabricate the second block1130.

The cell 1102 couples light bi-directionally between first/second ports1180-1182 and the third/fourth ports 1184-1186. For purposes ofillustration optical beams 1160-1162 are shown entering the cell at thefirst and second ports 1180-1182 respectively and exiting the cell asbeams 1178-1176 at the third and fourth ports 1184-1186 respectively.Propagation in the opposite direction is also supported. The cell isalso highly directional so that light propagating in one direction isindependent of the light propagating in the reverse direction. In theexample shown, beam 1160 enters the cell at port 1180 and beam 1162enters the cell at port 1182. Each beam is accepted onto either of twodistinct transmission (T₁) and reflection (R₁) delay paths 1170 and1166, 1168, 1172 respectively. These delay paths link the intensity beamsplitters 1164, 1174. The amount of light that is coupled from theinputs at ports 1180 and 1182 onto either delay path by each beam isdetermined by the beam path and the ratio or percentage of transmissionand reflection of the beam splitter 1164. The amount of light that iscoupled from the either delay path to the output at ports 1184 and 1186is determined by the beam path and the ratio or percentage oftransmission and reflection of the beam splitter 1174. The percentage oftransmission and reflection is an optical property that can be preciselyspecified. The intensity beam splitters 1164, 1174 thus serve ascouplers which provide configurable amounts of coupling andcross-coupling of incoming beams with either of the pair of delay paths.

The amount of delay on the T₁ and R₁ delay paths 1170 and 1166, 1168,1172 are θ_(r1) and θ_(R1) respectively. The delay of each path isdetermined by its corresponding optical path length. The optical pathlength of each path is the sum of product of the physical dimension andthe index of refraction of all the optical elements on each of the twodistinct delay paths. The delay difference for the cell is proportionalto the difference in the optical path lengths between the R₁ and T₁delay paths. The delay difference exhibits itself in the opticalproperties of the output beam 1176-1178. The output beams exhibit aninterference pattern produced by the re-coupling of the R₁ and T₁ delaypaths by the second of the beam splitters 1174. Each output beamcontains complementary periodic stop bands and pass bands of the otherwith center wavelengths the spacing between which is inversely relatedto the delay difference between the R₁ and T₁ delay paths. In otherwords the larger the delay difference the smaller the wavelength spacingwhich the optical filter cell can resolve. Output beam 1176 containspass bands with center wavelengths at even integer multiples of theperiodic interval established by the delay difference. Output beam 1178contains pass bands with center wavelengths at odd integer multiples ofthe periodic interval established by the delay difference.

The single cell 1102 nay serve as either or both a multiplexer or ademultiplexer depending on a propagation direction of the light.

For each of the optical filters discussed above it may be necessary toadjust the center wavelength and free spectral range of any given cellor set of delay paths. This can be accomplished by tilting of the cellabout the x axis normal to the propagation path, or by tilting each ofthe component within the cell resulting a net change of effectiveoptical path length difference. This will allow a coarse tuning to theappropriate free spectral range followed by a shifting, i.e. a finetuning, of the center wavelength of all the pass bands generated by eachcell or delay path.

FIG. 8C is an isometric view of an optical filter cell shown in FIG. 8Awith one of the optical elements, i.e. element 1120AB, configured foradjustment of the center wavelength and free spectral range of thefilter. Optical element 1120AB is shown is cleaved into a pair ofcomplementary wedges 1120A and 120B. As each wedge is moved in opposingdirections along the x axis the optical path length of delay path 1150is altered. This results in a shift in the center wavelength and achange in the free spectral range of the cell. Once the cell exhibitsthe desired center wavelength the wedges 1120A, 1120B are fixed relativeto the blocks 1120 and 1130.

FIG. 8D is an isometric view of an optical filter cell shown in FIG. 5Bwith one of the optical elements configured for adjustment the centerwavelength and free spectral range of the filter and with an alternatecoupler configuration. As discussed above in connection with FIG. 8C thecleaving of element 1120 into wedges 1120A and 1120B provides a meansfor shifting the center wavelength and adjusting the free spectral rangeof the cell.

FIG. 8D also introduces an alternate configuration for the blocks 1110and 1130 shown in FIGS. 8A-B. This alternate block configuration isshown in the context of intensity dependent beam splitting as introducedin FIG. 8B, and may be applied with equal advantage to the polarizationdependent beam splitting shown in FIG. 8A. The couplers 1164, 1174 aredefined on the corresponding external base faces of a pair of opposingblocks 1188 and 1190. Each block 1188, 1190 is configured with an upperangular portion on the corresponding external surfaces of which thereflectors 1112, 1132 respectively are defined.

FIG. 8E is an isometric view of an optical filter cell shown in FIG. 8Bwith one of the optical elements configured for adjustment of the centerwavelength and free spectral range of the filter and with an alternatecoupler configuration. As discussed above in connection with FIG. 8C thecleaving of element 1120 into wedges 1120A and 1120B provides a meansfor shifting the center wavelength and adjusting the free spectral rangeof the cell.

FIG. 8E also introduces still another configuration for the blocks 1110and 1130 shown in FIGS. 8A-B. This alternate block configuration is alsoshown in the context of intensity dependent beam splitting as introducedin FIG. 8B, and may be applied with equal advantage to the polarizationdependent beam splitting shown in FIG. 8A. In this embodiment of theinvention the block structure is dispensed with and each coupler 1164,1174 is defined on its own discrete substrate. Suitable substratesinclude any suitable transparent medium e.g. optical glass or asemiconductor. Similarly, reflectors 1112 and 1132 are rennin on theirown discrete substrate, which in this case does not need to betransparent. Each coupler and reflector is positioned with respect toone another by a suitably stable frame, not shown.

FIGS. 9A-B are isometric and end views respectively of a multi-cellimplementation of the optical filter cell shown in FIG. 8A. Two cells1100 and 1200 are shown coupled serially to one another in sequence.This concept of serially coupling cells allows an optical filter toexhibit a more complex transfer function than the simple sinusoidaloutput provided by the single cell shown in FIG. 8A. In this example thedelay paths provided by cell 1200 and their delay difference are largerthan the delay paths and delay difference provided by the cell 1100.This result can be achieved either by fabricating cell 1200 from thesame optical elements as cell 100 with an increase in the physicallength of the elements or by fabricating cell 1200 from optical elementswith higher indices of refraction than those of cell 100 thusmaintaining the same form factor for both cells.

The combination of first cell and subsequent cells can be looked at asestablishing by the difference between their delay paths the fundamentalsinusoidal harmonic for the sequence and higher order harmonics. In anembodiment of the invention this objective is achieved by designing oneof the cells in the sequence with a FSR corresponding with the desiredfundamental harmonic and with others of the cells designed with FSRswhich are integer fractions of the base FSR. The coefficients oramplitude of each harmonic are provided by varying the couplingratio/percentage/coefficients between a polarized input to a cell andthe P and S polarization axes of the cell as provided by thecorresponding polarization beam splitter. The coupling coefficients arevaried by tilting of a cell about the propagation path of a polarizedinput to each cell.

Cell 1100 includes the components described above in connection withFIG. 8A. Cell 1200 includes couplers 1214, 1234 employing polarizationdependent beam splitting linked by a pair of delay paths 1250 and 1246,1248, 1252. The cell 1200 includes opposing optical glass blocks 1210,1230 displaced from one another along the z axis with the opticalelement 1220 covering the span between the blocks. Optical glass block1210 is shown with a reflector 1212 and a polarization dependent beamsplitter 1214 displaced from each other in a direction defined by the yaxis. Optical glass block 1230 is shown with a reflector 1232 and apolarization dependent beam splitter 1234 displaced from each other in adirection defined by the y axis. The polarization dependent beamsplitters have “S” polarization axis which are aligned with one another.Between the couplers an optical element 1220 is shown. The variouscomponents are shown on top of base 1206.

Polarized beam 1140 has, in the example shown, a linearly polarizedinput with a vector 1144. This beam enters the cell 1100 at the firstport 1102, is reflected and transmitted by polarization beam splitter1114 onto one end of the pair of delay paths θ_(S1) and θ_(P1). At theopposite end of the delay paths reflection and transmission by thepolarization beam splitter 1134 produces a common output beam 1154 whichexits the cell 1100 at port 1104. Port 1104 of the first cell coupleswith port 1202 of the next cell 1200 in the sequence. Thus, the beam1154 output from the first cell enters the second cell 1200 where it isreflected and transmitted by polarization beam splitter 1214 onto oneend of the pair of delay paths θ_(S2) and θ_(P2). At the opposite end ofthe delay paths reflection and transmission by the polarization beamsplitter 1234 produces a common output beam 1254 which exits the cell1200 at port 1204, with orthogonal polarization vectors 1256-1258 withodd and even components, respectively. The process can be repeated toform a longer sequence of cells and a more complex optical filtertransfer function.

FIG. 9C is a side elevation view of the delay paths of the multi-cellimplementation shown in FIGS. 9A-B. The delay introduced into lightpassing along any delay path is a function of the optical pa length ofthe optical elements which comprise the delay path. Optical path length“L” of an optical element is expressed as the product of the physicaldistance “d” traversed by a beam propagating through the elementmultiplied by the index of refraction “n” the element. Where multipleoptical elements are involved the individual contributions to theoptical path length from the individual elements are summed. Forpurposes of the current invention optical elements include: a vacuum agas, a liquid, a solid or a plasma along the propagation path. The indexof refraction of a medium identifies ratio of the speed of light in avacuum to that of light in the medium. Where the optical path lengthvaries as here between two delay paths one path is said to befaster/slower than the other and there is said to be a delay differencebetween the two.

Beam 1140 propagates through the first cell 1100 across delay pathsθ_(P1) and θ_(S1) and through the second cell 1250 across delay pathsθ_(P2) and θ_(S2). Delay path θ_(P1) comprises optical elements definedby optical path length L₁₅-L₁₇. Delay path θ_(S1) comprises opticalelements defined by optical path lengths L₁₀-L₁₄. Delay path θ_(P2)comprises optical elements defined by optical path length L₂₅-L₂₇. Delaypath θ_(S2) comprises optical elements defined by optical path lengthsL₂₀-L₂₄. In the embodiment shown the optical elements defined by opticalpath lengths L₁₂-L₂₂ include air/gas/vacuum. The remaining opticalelements may be fabricated from various types of optical glassincluding: BKx, fused silica, SFx. By proper design of delay paths thefundamental and higher order harmonics for the optical filter may beestablished.

The delay for the delay paths θ_(P1) and θ_(S1) in the first filter 1100are expressed as a function of the optical path lengths of each path inthe follow Equations 4-5 respectively. $\begin{matrix}{\theta_{S1} = {\left( {2\quad\pi\quad\frac{v}{c}} \right)\left( {\sum\limits_{i = 1}^{i = I}{d_{i}\quad n_{i}}} \right)}} & \text{Equation~~~4} \\{\theta_{P1} = {\left( {2\quad\pi\quad\frac{v}{c}} \right)\left( {\sum\limits_{j = 1}^{j = J}{d_{j}\quad n_{j}}} \right)}} & \text{Equation~~~5}\end{matrix}$where c and v are the frequency and velocity of light in free space andwhere I and J are the number of optical elements which make up the delaypaths with delays θ_(S1) and θ_(P1) respectively. For each of the Ioptical elements: vacuum, gas, plasma, liquid or solid which make up thedelay path θ_(S1) the i^(th) element has a physical length d_(i) and anindex of refraction n_(i). For each of the J optical elements: vacuum,gas, plasma, liquid or solid which make up the delay path θ_(P1) thej^(th) element has a physical length d_(j) and an index of refractionn_(j). Optical elements include the optical coatings associated withpolarization or intensity dependent beam splitters, which alsocontribute to optical pathlength and phase accumulations.

The delay difference between the two paths is expressed in Equation 6.$\begin{matrix}{{\theta_{S1} - \theta_{P1}} = {\left( {2\quad\pi\quad\frac{v}{c}} \right)\left( {{\sum\limits_{i = 1}^{i = I}{d_{i}\quad n_{i}}} - \left( {\sum\limits_{j = 1}^{j = J}{d_{j}\quad n_{j}}} \right)} \right)}} & \text{Equation~~~6}\end{matrix}$

The delay difference for the cell is proportional to the difference inthe optical path lengths between the S and P delay paths. Similarconsiderations apply in determining the delays and delay differences forthe pair of delay paths in the second cell 1200.

The optical p length difference between the two delay paths in a cell,corresponds inversely with the free spectral range (FSR) generated bythe cell as evidenced in the orthogonal vector components of the outputbeam from the cell. This relationship is set forth in the followingEquation 7. $\begin{matrix}{{FSR} = {\left( \frac{c}{{L_{I} - L_{J}}} \right) = {\left( \frac{c}{{{\sum\limits_{i = 1}^{i = I}{d_{i}\quad n_{i}}} - {\sum\limits_{j = 1}^{j = J}{d_{j}\quad n_{j}}}}} \right) = {2\quad\pi\quad\frac{v}{\left( {\theta_{s} - \theta_{p}} \right)}}}}} & \text{Equation~~~7}\end{matrix}$where L_(I) and L_(I) are the total optical path length of the I and Jelements which make up the corresponding delay paths θ_(S1), and θ_(P1).

For an optical mux/demux the FSR should be a constant perhaps equal tothe channel spacing, e.g., 100 GHz. Using Equation 7 the delaydifference required to generate this result can be determined, and fromit a solution to the optical path lengths for each of the delay paths.

FIG. 9D is a side elevation view of the variable coupling between cellsof the multi-cell implementation shown in FIGS. 9A-B. Coupling is usedto control the amount an input of polarized light that will be projectedonto the S and P delay paths of a corresponding cell. Three couplingviews 1260, 1262 and 1264 are shown at appropriate locations at theinput to cell 1100, the interface between cells 1100 and 1200 and at theoutput of cell 1200 respectively. The three views 1260-1264 are from theoutput port 1204 looking back along the propagation path of the inputbeam 1140. In the first of the coupling views 1260, the polarized inputis shown with a single input vector “T” and the orthogonal polarizationaxis P1 and S1 of the first cell 1100 are shown. The input I may includeorthogonal input vectors. The amount of light that is coupled ontoeither delay path in the first cell is determined by the angle φ₁ of theincoming beam vector with the S polarization axis of the cell. In theexample shown light from polarization vector 1144 in amountsproportionate to sin (φ₁) and cos(φ₁) will couple to the P and S delaypaths respectively. Rotation of the cell about the propagation path ofthe bean 1140 can be used to vary the coupling percentages or ratiosbetween the incoming light and the P and S delay paths. In the nextcoupling view 1262, light from the output port of cell 1100 is coupledwith the input port of cell 1200. The orthogonal polarization vectorsP₁, S₁ present in the output of the first cell are shown along with theorthogonal polarization vectors P₂, S₂ which are defined by the couplerof the next cell in the sequence, i.e. cell 1200. The amount of lightthat is coupled onto either delay path in the second cell is determinedby the angle φ₁ between the two sets of orthogonal vectors for the inputand the cell 1200. The last coupling view 1264, shows the orthogonalpolarization vectors, P₂, S₂ present in the output of the second cellalong with a second set of orthogonal polarization vectors P_(o), S_(o).This last orthogonal vector set is used to represent output optics usedto separate the orthogonal vectors within the single output beam intotwo discrete beams (not shown). The amount of light coupled onto theoutput beams is defined by an angle φ₃ between the two sets oforthogonal vectors.

FIG. 9E shows the individual transforms 1288 associated with each of thefour distinct delay paths from the input port 1102 to the output port1204. The number of discrete paths in a multi-cell sequence of N cellswith two delay paths between each equals 2^(N). For two cells there are2¹ or 4 discrete paths between an input and an output. The first ofthese paths is along delay paths θ_(S1) and θ_(S2) in the fit cell 1100and the second cell 1200 respectively. The second of these paths isalong delay paths θ_(S1) and θ_(P2). The third of these paths is alongdelay paths θ_(P1) and θ_(S2). The fourth of these paths is along delaypaths θ_(P1) and θ_(P2). The transfer function for the optical filter insingle or sequential cell embodiments is the sum of the individualtransfer functions associated with each discrete path from input tooutput. Transfer functions: 1288, 1290, 1292, 1294 are shown for the1^(st) to 4 th paths discussed above. Each transfer function includestwo terms 1296-1298. The first term 1296 corresponds to a coefficient ina Fourier series with the coefficient magnitude proportional to theproduct of the coupling or cross coupling coefficients along theparticular path. The second term 1298 corresponds to the frequencycomponent associated with that coefficient. The frequency componentcorresponds with the sum of the delays along a corresponding path. Thisin turn corresponds with the optical path lengths along each path. Thesum of all the transfer functions forms a truncated Fourier series whichfully defines the optical filter.

In an embodiment of the invention a multi-cell design includes: a firstcell of 100 GHz FSR and a 2^(nd) cell of 50 GHz FSR can be used to forma polarization type square top comb filters. This filter can split anoptical stream with 50 GHz channel spacing into two separate opticalstream with odd and even 100 GHz charnel spacing respectively orcombining two optical stream with 100 GHz odd and even channel spacingrespectively into an optical stream with 50 GHz channel spacing. The1^(st) angle φ₁ can substantially equal to 45 degree and 2nd angle φ₂can substantially equal to (45+22.5) degree. Similarly, a fist cell of100 GHz FSR and a cell of 50 GHz FSR can be used to form a intensitytype of square top comb filters. The 1^(st) splitting ratio equalssubstantially to 50/50% and the 2^(nd) splitting ratio equalsCos²(45+22.5^(o))/Sin²(45+22.5^(o)). In still another embodiment of theinvention a first stage with a plurality of cells and a second stagewith a plurality of cells can be coupled together to further clean upthe signal. In a multi-cell embodiment a square top filter function maybe achieved by choosing one cell with the base FSR and with the FSRs ofthe remaining cells standing in relation to the FSR of the base cell asinteger fractional multiples thereof.

Further teachings on sequentially coupled optical filter cells may befound in either of the two following references. See S. E. Harris etal., Optical Network Synthesis Using Birefringent Crystals. JOURNAL OFTHE OPTICAL SOCIETY OF AMERICA, VOLUME 54, Number 10, October 1964 for ageneral discussion of transfer functions related to birefringentcrystals, which is hereby incorporated by reference as if fully setforth herein. See C. R Henry et al. U.S. Pat. No. 5,596,661 entitled“Monolithic Optical Waveguide Filters based on Fourier Expansion” issuedon Jan. 21, 1997 for a general discussion of transfer functions relatedto waveguides, which is hereby incorporated by reference as if fully setforth herein.

Passive Thernal Stabilization

The typical application of optical filters constructed using the abovetechniques is an optical mux/demux In order for that device to functionproperly it must create the desired stop bands and pass bands for theodd and even channels which it separates. For current telecommunicationapplications the filter would be designed with a constant FSR perhapsequal to the channel spacing, e.g., 100 GHz. An optical filter with thisFSR would generate the required stop bands and pass bands in each of theorthogonal polarization vectors present on the output. One of theorthogonal output vectors would contain the pass bands associated withthe center wavelengths of the odd channels. The other of the orthogonaloutput vectors would contain the pass bands associated with the centerwavelengths of the even channels.

Temperature variations in a mux/demux that may effect the performancemay result from the environment or from the power transmitted throughthe device. This can result in the periodic odd and even pass bandsgenerated by the optical filter moving out of alignment with theselected grid, e.g., the ITU grid. This is primarily because the centerwavelength of a pass band slips with temperature. This misalignmentresults in attenuation of signal strength, cross talk and ultimatelyloss of transmission/reception capability until the optical filterreturns to its original temperature. In practice therefore, the opticalfilters and devices fabricated therefrom must be thermally stable acrossa rage of temperatures.

One solution is to flatten the pass bands of the filter. Multi cellfilter designs such as those discussed above allow the pass bands toexhibit higher order harmonics in the form of non-sinusoidal pass bandprofiles, a.k.a “flat tops” (See FIG. 11). The close spacing between thechannels in a WDM, makes it desirable to design a WDM with flat passbands in order to increase the error tolerance to temperature inducedshifts in the pass bands. Even with flat top filter profiles temperaturestabilization is still required due to the precise telecommunicationchannel spacing.

One solution is to actively stabilize the temperature of themultiplexer/de-multiplexer using a heater or cooler and a dosed loopfeedback of temperature or wavelength. This solution can be expensiveand any increase the form factor of the mux/demux. Nevertheless, thecurrent invention may be practiced with active temperaturestabilization. A possible benefit to active temperature stabilization isthat the optical elements which nuke up each pair of delay paths may allbe fabricated from a common medium with identical indices of refractionand thermal expansion coefficient.

Although capable of being utilized in systems with active temperaturestabilization, the current invention is capable of providing temperaturestability for the optical filters without active temperature controlwhere appropriate. This greatly enhances the precision of the devices orsystems fabricated therefrom and reduces system cost. The currentinvention is capable of providing passive temperature stabilization ofan optical device, through proper selection and design of the opticalelements which form each pair of delay paths so that the delaydifference for each pair of delay paths and hence the system as a wholeremain constant. Since the delay difference is directly related to thedifference in the optical path lengths this invention provides thermalstabilization of the delay difference. As opposed to prior art designthat use a single medium for each pair of delay p, the current inventionprovides at least one of the delay paths with two optical elements eachof which exhibits a different optical path length response totemperature. Typically this takes the form of optical elements withdifferent thermal optic coeffients.

The system is designed so that d(L₁-L_(J))/dT equals substantially zero.This latter condition is satisfied when the derivative of thedenominator in Equation 7 substantially equals zero as set forth in thefollowing Equation 8: $\begin{matrix}{\frac{\mathbb{d}\left( {L_{I} - L_{J}} \right)}{\mathbb{d}T} = {\frac{\mathbb{d}\left( {{\sum\limits_{i = 1}^{i = I}{d_{i}\quad n_{i}}} - {\sum\limits_{j = 1}^{j = J}{d_{j}\quad n_{j}}}} \right)}{\mathbb{d}T} = {{{\sum\limits_{i = 1}^{i = I}\left( {{d_{i}\beta_{i}} + {\alpha_{i}\quad n_{i}d_{i}}} \right)} - {\sum\limits_{j = 1}^{j = J}\left( {{d_{j}\beta_{j}} + {\alpha_{j}\quad n_{j}d_{j}}} \right)}} \approx 0}}} & \text{Equation~~~8}\end{matrix}$where a_(i) and a₄ are the thermal expansion coefficients for eachoptical element which form the S and P delay paths respective in eachcell and where β_(i) and β_(j) are the thermal optic coefficients forthe temperature induced change in the refractive index for each elementin the S and P delay paths respectively.

The following Table 3 shows various relevant optical parameters for someoptical mediums which may be used to fabricate the optical elementswhich make up each pair of delay paths.

TABLE 3 Fused 1 @1550 nm vacuum Air BK7 SF5 Silica BaK1 LaSFN30 2 n 11.00027 1.50066 1.64329 1.44409 1.55517 1.77448 3$\beta = {\frac{\mathbb{d}n}{\mathbb{d}T} \times 10^{- 6}}$ 0 0*   0.907465 1.407 13.7 0.066 2.293 4 α × 10⁻⁶ 0 0*    5.1 8.2 0.52 7.6 6.2*assuming constant volume

For each material its refractive index at 1550 nm is set forth in row 2respectively. The change in refractive index n as a function oftemperature β is set forth in rows 3. Row 7 sets forth the coefficientof thermal expansion a for the medium.

The selection of physical length of each optical components can bedetermined by solving Equation 4 and 5 together. For example, for cell1100, 1^(st) coupler 1110, 2^(nd) coupler 1130 and spacer 1150 can bemade of fused silica. The 1^(st) beam splitting surface of prism 1110forms 35 degree angle with respect to bottom surface of the coupler. Fora 100 GHz FSR and thermally compensated cell, L16=2.862 mm and its widthis FIG. 2 c is 3.014 mm. The spacer length L10=1.8475 mm. 2^(nd) coupler1130 is identical to 1^(st) coupler 1110.

FIG. 10A is an isometric side view of an optical filter 1300 constructedfrom a series of delay paths coupled by intensity dependent beamsplitters. Such a filter could be fabricated strictly by repetition ofthe cell structure shown in FIG. 8B. In this embodiment of the inventionhowever, intermediate couplers are configured in a single coupling blockwith a pair of reflectors. Each intermediate coupling block couples anadjacent pair of delay paths.

Three coupler/reflector blocks 1110, 1310, 1330 are shown with a firstdelay path pair θ_(R1), θ_(T2) also referenced as 1168-1170 respectivelyand a second delay path pair θ_(R2), θ_(T2) also referenced as 1322,1318 respectively, between them to form a sequence of delay paths. Thesequence of delay paths allows, as discussed above, an optical filter toexhibit a more complex transfer function than the simple sinusoidaloutput provided by the single cell shown in FIG. 8B. The second pair ofdelay paths θ_(R2), θ_(T2) are shown with a delay difference larger thanthe first pair of delay paths. For purposes of example the physicaldimension of the second pair of delay paths is larger than the firstpair. If the indices of refraction of the optical elements of the secondset of delay paths are increased the physical dimension required togenerate the larger delay difference will decrease. The first delay pathpair can establish by the difference between its delay paths thefundamental sinusoidal harmonic for the sequence with the second delaypath pair providing higher order harmonics. The coefficients oramplitude of each harmonic are provided by varying the couplingratio/percentage/coefficients between the reflection and transmissiondelay paths within each delay path pair. The coupling coefficients arevaried by varying the reflection and transmission ratios for eachintensity beam splitter 1164, 1314 and 1334 with the sequence.

The first coupler block includes intensity beam splitter 1164 andreflector 1112. The intensity beam splitter accepts input from either orboth of beans 1160-1162 at the first and second ports 1180-1182respectively. Optical element 1120 bridges the gap betweencoupler/reflector block 1110 and the next coupler/reflector block 1310in the sequence. Coupler/reflector block 1310 includes intensity beamsplitter 1314 and reflectors 1312, 1316. The intensity beam splittercouples the inputs/outputs from the first pair of delay paths θ_(R1),θ_(r1) to the output/inputs of the second delay path pair θ_(R2) θ_(T2).The reflectors 1312, 1316 handle the redirection of the delay pathsθ_(R1) , θ_(R2) respectively. Optical element 1320 bridges the gapbetween coupler/reflector block 1310 and the next coupler/reflectorblock 1330 in the sequence. Coupler/reflector block 1330 includesintensity beam splitter 1334 and reflectors 1332, 1336. The intensitybeam splitter couples the inputs/outputs from the second pair of delaypaths θ_(R2), θ_(T2) to the third and fourth ports 1338-1340. Thereflector 1336 handles the redirection of the delay path θ_(R2). Eachcoupler reflector block may be fabricated from optical glass in the samemanner discussed above in connection with FIG. 8B. The variouscomponents are shown on top of base 1302.

Optical beams 1160-1162 input at ports 1180-1182 respectively, traversethe sequence of delay paths to exit as two discrete optical beams1350-1352 at ports 1338-1340 respectively. The basic structure shownhere can be continued to form a longer sequence of cells and a morecomplex optical filter transfer function.

FIG. 10B is a side elevation view of the delay paths of the multi-cellimplementation shown in FIG. 10A The delay introduced into light passingalong any delay path is a function of the optical path length of thevarious optical elements on the delay path For purposes of the currentinvention optical elements include: a vacuum, a gas, a liquid, a solidor a plasma along the propagation path Beams 1160-1162 propagate throughthe first pair of delay paths θ_(R1) and θ_(T1) and the second pair ofdelay paths θ_(R2) and θ_(T2). Delay path θ_(T1) comprises opticalelements defined by optical path length L₁₀-L₁₁ and L₁₅. Delay pathθ_(R1) comprises optical elements defined by optical path lengthsL₁₂-L₁₆. Delay path θ_(T2) comprises optical elements defined by opticalpath lengths L₂₅-L₂₇. Delay path θ_(R2) comprises optical elementsdefined by optical path lengths L₂₀-L₂₄. In the embodiment shown theoptical elements defined by optical lengths L₁₄ and L₂₂ includeair/gas/vacuum. The remaining optical elements may be fabricated fromvarious types of optical glass including: BK7, fused silica, SF5. Byproper design of delay paths the fundamental and higher order harmonicsfor the optical filter may be established.

The delay for the delay paths θ_(R1) and θ_(r1) are expressed as afunction of the optical path lengths of each path as discussed above inequations Equations 4-5, with θ_(R1) substituted for θ_(S1) in Equation4 and with θ_(r1), substituted for θ_(P1), respectively.

The delay difference between the two delay paths is calculated in thesame manner as shown in Equation 6 above. The delay difference for thecell is proportional to the difference in the optical path lengthsbetween the reflection (R) and transmission A) delay paths. Similarconsiderations apply in determining the delays and delay differences forthe second pair of delay paths θ_(R2) and θ_(T2).

The optical path length difference between the two delay paths in adelay path pair corresponds inversely with the free spectral range (FSR)of the cell as evidenced in the pass bands and stop bands in the twooutput beams 1350-1352 from the cell. This relationship is set forth anddiscussed in Equation 7 above with L₁ and L₁ representing in thisembodiment the total optical path length of the I and J elements whichmake up the corresponding delay paths θ_(R1), and θ_(R1). Now, for anoptical mux/demux a condition to be satisfied is that the FSR be aconstant perhaps equal to the channel spacing, e.g., 100 GHz UsingEquation 7 the delay difference required to generate this result can bedetermined, and from it a solution to the optical path lengths for eachof the delay paths.

FIG. 10C is a side elevation view of the variable coupling between cellsof the multi cell implementation shown in FIG. 10A Coupling is used tocontrol the amount an input of polarized light that will be projectedonto the R and T delay paths. The coupling values for the intensity beamsplitters 1164, 1314, and 1318 are [R₁, T₁], [R₂, T₂] and [R₃,T₃]respectively.

FIG. 10D shows the individual transforms associated with each of theoptical paths for input beam 1160 from the input port 1180 (See FIG.10A) to the output port 1350. The number of discrete paths in amulti-cell sequence of N cells with two delay paths in each delay pathpair, equals 2^(N) as discussed above in connection with FIGS. 9D-E. Forthe embodiment shown in FIGS. 10A-C with two pairs of delay paths thereare 2² or 4 discrete paths between any one of the two port serving asinputs and any one of the two output ports serving as output. Dealingwith beam 1160 as an input at the first port 1180 (See FIG. 10A) andbeam 1350 as an output at port 1338 (See FIG. 10A) there are 4 discretedelay paths from input to output. These paths are θ_(R1)>θ_(R 2),θ_(R1)>θ_(T2), θ_(T1)>θ_(T2), and θ_(T1)>θ_(R2). The transfer functionfor the optical filter in singe or sequential cell embodiments betweenany input and output port is the sum of the individual transferfunctions associated with each discrete path from input to output.Transfer functions: 1388, 1390, 1392, 1394 are shown for the 1^(st) to4^(th) paths discussed above. Each transfer function includes two terms1396-1398. The first term 1396 corresponds to a coefficient in a Fourierseries with the coefficient magnitude proportional to the product of thecoupling or cross coupling coefficients along the particular path. Thesecond term 1398 corresponds to the frequency component associated withthat coefficient. The frequency component corresponds with the sum ofthe delays along a corresponding path This in turn corresponds with theoptical path lengths along each path. The sum of all the transferfunctions forms a truncated Fourier series which fully defines theoptical filter.

Thermal stabilization of the delay pairs is effected in this embodimentof the invention in the same manner as discussed above in connectionwith Equation 8, for all the optical elements which make up each of thedelay paths.

FIG. 11 is a graph showing the pass bands and stop bands associated witha specific filter transform which may be achieved using the opticalfilters of the current invention. In the example shown the envelopeassociated with six narrowly spaced, i.e., 100 GHz or 0.8 nm WDMchannels. The odd channels 1400, 1404, 1408 are shown in solid line. Theeven channels 1402, 1406 are shown in dashed line. The precise centerfrequencies of each channel are specified by standard settingorganizations such as the International Telecommunications Union (TU).These center frequencies are set forth as part of a wavelength gridwhich defines the center frequencies and spacing between channels. Thepass bands exhibit flat tops which may be preferred because each channelis subject to shifting around the center frequency and a flat top avoidsattenuation of a channel subject to such shifting. Shifting may becaused by any one of a number of factors including temperature,inter-channel interference and polarization mode dispersion. The fat topprofile is achieved by the sequencing of optical filters as shown ineither of FIG. 9A or 10A, to provide higher order harmonics.

In alternate embodiments of the invention the cells and serially coupledcells may be fabricated on a common semi-conductor substrate. Thevarious components: reflectors, couplers, and optical elements may befabricated using a combination of etching and deposition techniques wellknow in the semiconductor industry.

The foregoing description of preferred embodiments of the invention hasbeen presented for purposes of illustration and description. It is notintended to be echaustive or to limit the invention to the precise formsdisclosed: Obviously many modifications and variations wig be apparentto practitioners skilled in this art.

1. A method for splitting or combining optical signals, comprising:subjecting odd and even channel sets to a first set of phaseretardations corresponding with odd integer multiples of half awavelength for each center wavelength associated with a selected one ofthe odd channel set and the even channel set and corresponding withinteger multiples of a full wavelength for each center wavelengthassociated with the other of the odd channel set and the even channelset; and subjecting the odd and even channel sets to a second set ofphase retardations corresponding with integer multiples of a fullwavelength for each center wavelength associated with the selected oneof the odd channel set and the even channel set and corresponding withodd integer multiples of half a wavelength for each center wavelengthassociated with the other one of the odd channel set and the evenchannel set.
 2. The method of claim 1, wherein subjecting the odd andeven channel sets to a first set of phase retardations comprises:filtering the selected one of the odd channel set and the even channelset with a first comb filer function; and filtering the other of the oddchannel set and the even channel set with a second comb filter functiondistinct from the first comb filter function.
 3. The method of claim 2,wherein subjecting the odd and even channel sets to a second set ofphase retardations comprises: filtering the selected one of the oddchannel set and the even channel set with the second comb filterfunction; and filtering the other of the odd channel set and the evenchannel set with the first comb filter function to broaden stopbandsbetween adjacent odd channels and between adjacent even channels.
 4. Themethod of claim 3, wherein each of the filtering acts further comprisesasymmetrically coupling the optical signals onto at least one pair offast and slow delay paths to effect the broadening of the stopbands. 5.The method of claim 1, wherein subjecting the odd and even channel setsto a first set of phase retardations comprises: filtering the selectedone of the odd channel set and the even channel set with a first combfilter function that exhibits a primary periodicity corresponding totwice a spacing between adjacent channels; and filtering the other ofthe odd set of channels and the even set of channels with a second combfilter function that exhibits both the primary periodicity and aresidual periodicity substantially equal to the primary periodicity andshifted in phase with respect thereto by π.
 6. The method of claim 5,wherein subjecting the odd and even channel sets of a second set ofphase retardations comprises: filtering the selected one of the oddchannel set and the even channel set with the second comb filterfunction; and filtering the other of the odd channel set and the evenchannel set with the first comb filter function to broaden stopbandsbetween adjacent odd and adjacent even channels.
 7. The method of claim1, further comprising splitting the odd channel sets from the evenchannel sets after subjecting the odd and even channel sets to the firstset of phase retardations and before subjecting the odd and even channelsets to the second set of phase retardations.
 8. The method of claim 1,further comprising combining the odd channel sets with the even channelsets after subjecting the odd and even channel sets to the first set ofphase retardations and before subjecting the odd and even channel setsto the second set of phase retardations.
 9. The method of claim 1,further comprising: receiving the odd channel set and the even channelset at a single input port before subjecting the odd and even channelsets to the first set of phase retardations; and outputting the oddchannel set at a first output port after subjecting the odd and evenchannel sets to the second set of phase retardations; and outputting theeven channel set at a second output port after subjecting the odd andeven channel sets to the second set of phase retardations, such that theodd channel set has been split from the even channel set.
 10. The methodof claim 1, further comprising: receiving the odd channel set at a firstinput port before subjecting the odd and even channel sets to the firstset of phase retardations; receiving the even channel set at a secondinput port before subjecting the odd and even channels sets to thesecond set of phase retardations; and outputting the odd channel set andthe even channel set at a single output port after subjecting the oddand even channel sets to the second set of phase retardations, such thatthe odd channel set and the even channel set have been combined.
 11. Ina multiplexer that process optical signals, including an odd channel setand an even channel set, a method for combining the odd channel set andthe even channel set, comprising: receiving the odd channel set at afirst input port of the multiplexer; receiving the even channel set at asecond input port of the multiplexer, subjecting the odd and evenchannel sets to a first set of phase retardations corresponding with oddinteger multiples of half a wavelength for each center wavelengthassociated with a selected one of the odd channel set and the evenchannel set and corresponding with integer multiples of a fullwavelength for each center wavelength associated with the other of theodd channel set and the even channel set; combining the odd channel setand the even channel set; subjecting the odd and even channel sets to asecond set of phase retardations corresponding with integer multiples ofa full wavelength for each center wavelength associated with theselected one of the odd channel set and the even channel set andcorresponding with odd integer multiples of half a wavelength for eachcenter wavelength associated with the other one of the odd channel setand the even channel set; and outputting the odd channel set and theeven channel set at a single output port of the multiplexer.
 12. Themethod of claim 1, wherein subjecting the odd and even channel sets to afirst set of phase retardations comprises: filtering the selected one ofthe odd channel set and the even channel set with a first comb filerfunction; and filtering the other of the odd channel set and the evenchannel set with a second comb filter function distinct from the firstcomb filter function.
 13. The method of claim 12, wherein subjecting theodd and even channel sets to a second set of phase retardationscomprises: filtering the selected one of the odd channel set and theeven channel set with the second comb filter function; and filtering theother of the odd channel set and the even channel set with the firstcomb filter function to broaden stopbands between adjacent odd channelsand between adjacent even channels.
 14. The method of claim 11, whereinsubjecting the odd and even channel sets to a first set of phaseretardations comprises: filtering the selected one of the odd channelset and the even channel set with a first comb filter function thatexhibits a primary periodicity corresponding to twice a spacing betweenadjacent channels; and filtering the other of the odd set of channelsand the even set of channels with a second comb filter function thatexhibits both the primary periodicity and a residual periodicitysubstantially equal to the primary periodicity and shifted in phase withrespect thereto by π.
 15. The method of claim 14, wherein subjecting theodd and even channel sets of a second set of phase retardationscomprises: filtering the selected one of the odd channel set and theeven channel set with the second comb function; and filtering the otherof the odd channel set and the even channel set with the first combfilter function to broaden stopbands between adjacent odd and adjacenteven channels.
 16. In a demultiplexer that process optical signals,including an odd channel set and an even channel set, a method forsplitting the odd channel set from the even channel set, comprising:receiving the odd channel set and the even channel set at a single inputport of the demultiplexer; subjecting the odd and even channel sets to afirst set of phase retardations corresponding with odd integer multiplesof half a wavelength for each center wavelength associated with aselected one of the odd channel set and the even channel set andcorresponding with integer multiples of a full wavelength for eachcenter wavelength associated with the other of the odd channel set andthe even channel set; splitting the odd channel set from the evenchannel set; subjecting the odd and even channel sets to a second set ofphase retardations corresponding with integer multiples of a fullwavelength for each center wavelength associated with the selected oneof the odd channel set and the even channel set and corresponding withodd integer multiples of half a wavelength for each center wavelengthassociated with the other one of the odd channel set and the evenchannel set; and outputting the odd channel set from a first output portof the demultiplexer; and outputting the even channel set from a secondoutput port of the demultiplexer.
 17. The method of claim 16 whereinsubjecting the odd and even channel sets to a first set of phaseretardations comprises: filtering the selected one of the odd channelset and the even channel set with a first comb filer function; andfiltering the other of the odd channel set and the even channel set witha second comb filter function distinct from the first comb filterfunction.
 18. The method of claim 17, wherein subjecting the odd andeven channel sets to a second set of phase retardations comprises:filtering the selected one of the odd channel set and the even channelset with the second comb filter function; and filtering the other of theodd channel set and the even channel set with the first comb filterfunction to broaden stopbands between adjacent odd channels and betweenadjacent even channels.
 19. The method of claim 16, wherein subjectingthe odd and even channel sets to a first set of phase retardationscomprises: filtering the selected one of the odd channel set and theeven channel set with a first comb filter function that exhibits aprimary periodicity corresponding to twice a spacing between adjacentchannels; and filtering the other of the odd set of channels and theeven set of channels with a second comb filter function that exhibitsboth the primary periodicity and a residual periodicity substantiallyequal to the primary periodicity and shifted in phase with respectthereto by π.
 20. The method of claim 19, wherein subjecting the odd andeven channel sets of a second set of phase retardations comprises:filtering the selected one of the odd channel set and the even channelset with the second comb filter function; and filtering the other of theodd channel set and the even channel set with the first comb filterfunction to broaden stopbands between adjacent odd and adjacent evenchannels.